The present invention relates to flight control systems for rotary-wing aircraft with force feedback (active) collective systems, and more particularly to a flight control system which provides a low pilot-workload collective control system.
Demand for low pilot-workload helicopters continues to increase. With the establishment of ADS-33 as the dominant handling qualities specification, the rotorcraft industry has been pursuing various methods to provide advanced flight control features. One primary method of compliance with ADS-33 is employment of a Fly-By-Wire (FBW) flight control system to decouple pilot inputs and aircraft dynamics.
Several ADS-33 requirements relate to high vertical stability and precise command of vertical velocity. Unlike cyclic control, which in FBW system can be a “unique trim” controller (i.e. cyclic controller position does not directly represent swash plate angle), collective control has to maintain a more direct relationship to collective blade pitch. This piloting requirement complicates implementation of a collective controller with vertical augmentation. Below is a summary of current methods that implement additional vertical augmentation.
One current method that implements additional vertical augmentation includes the addition of a trim actuator to the collective axis as is typical of autopilots and some FBW systems. The pilot utilizes a trim actuator control such as a knob separate from the collective controller to enter a desired vertical velocity, altitude, etc. This method provides limited benefit to handling qualities, since the pilot does not directly command desired aircraft state. Should the pilot command collective directly, the trim actuator is overridden. This method necessarily limits slew rate of the trim actuator to prevent failure propagation.
Another current method that implements additional vertical augmentation includes the addition of a trim actuator and an inner loop servo typical of some autopilots and some FBW systems. This method is similar to the method described above, except it includes an additional actuator (in case of a mechanical flight control system) or an additional collective feed-forward path (in case of a FBW system). Low frequency commands are passed to the trim actuator and high frequency commands are passed to the inner-loop. This method provides some additional vertical augmentation when pilot is restraining/moving collective, but overall still suffers from the drawbacks described above.
Another current method that implements additional vertical augmentation includes the addition of a trim actuator and an additional separate vertical controller. In this method, the pilot moves the collective controller to input blade pitch commands directly into the rotor system. The pilot then moves a sidearm controller up and down to command aircraft vertical speed (instead of direct command of blade pitch). This method does meet ADS-33 requirements, but requires coordination between left and right hand, since small inputs are made with the right hand (into the sidearm controller) and large inputs are made with the left hand (collective controller). Adding this degree of freedom to the sidearm controller may also be prone to coupling which complicates single axis inputs. The pilot also has to be sure that the collective controller is unrestrained during inputs into the sidearm controller since inputs into the sidearm controller (through the FBW system) commands the trim actuator and displaces the collective controller.
Another current method that implements additional vertical augmentation utilizes an active inceptor system such as Boeing's Helicopter Advanced Control Technology (HACT) simulation This system allows force feedback, position feedback and some feedback on vertical aircraft state, but does not use force command as a pilot input. Instead, a “split-detent” feature where the detent is “stretched” or split operates to cue the pilot to two different aircraft conditions. This allows “level-flight” cueing, but if the pilot desires to command vertical speed, a displacement input is required. During this displacement input, additional augmentation is not provided. Since force is not utilized to command a different aircraft state, additional vertical axis augmentation is not provided when the pilot is flying “in-the-loop.”
Accordingly, it is desirable to provide force feedback as commanded by a flight control system such that pilots may seamlessly command vertical speed, flight path angle or directly change collective blade pitch.